1. Field of the Invention
The present invention relates to a process which converts biomass to hydrocarbons. Specifically, it relates to a process for producing hydrocarbon fractions such as liquefied petroleum gas (LPG), naphtha, and middle distillate fuels, such as jet, kerosene, and diesel, from fatty acids and glycerides.
2. Brief Description of the Related Art
Biomass is a renewable alternative to fossil raw materials in production of liquid fuels and chemicals. Development of more efficient biomass conversion processes for better quality fuel products is considered a key step toward wider use of renewable fuels.
Several prior art processes for producing fuels or fuel additives from starting materials such as plants and animals are known. U.S. Pat. No. 4,992,605 to Craig and Soveran (1991) discloses hydrodeoxygenation of vegetable oils to C15-C24 n-paraffins. The inventors point out that the high freeze point of this composition limits its use to that of additive for improving diesel fuel cetane rating. U.S. Pat. No. 5,705,722 to Monnier and co-inventors (1998) shows that the Craig and Soveran invention can be applied to tall oil, animal fats, and restaurant greases.
Swedish Patent 9700149 to Aalto and co-inventors (1997) teaches that the n-paraffins derived from hydrodeoxygenation of vegetable oils may be hydroisomerized to produce a composition suitable for direct use as diesel fuel. However the hydroisomerization of n-paraffins to native boiling range iso-paraffins is equilibrium limited and thus the product of n-paraffin hydroisomerization will always contain unisomerized n-paraffins. Presence of unisomerized C17 plus n-paraffins at even low concentrations can have a detrimental effect on the low temperature properties of the fuel. Referring to an example provided by Aalto and co-inventors, the cloud point of a middle distillate fuel with only 13% unisomerized C17 plus n-paraffins is −12° C. (typical Winter diesel cloud point specification is −22° C. maximum).
U.S. Pat. No. 7,232,935 to Jakkula and co-inventors (2007) shows that the hydrodeoxygenation and hydroisomerization may be conducted in a counter-current flow configuration to reduce hydrodeoxygenation catalyst exposure to water, CO, CO2, and hydroisomerization catalyst exposure to H2S and NH3. Although such a counter-current reactor design is expected to extend catalyst life, it does not address the low temperature performance issues associated with presence of unisomerized C17 plus n-paraffins in the diesel product.
To this end, although processes of the existing art utilize biomass to produce paraffinic biofuels, further improvements are desirable to provide new processing methods to make low cloud point middle distillate fuels.